The present invention relates to a novel nuclear fusion reactor, and, more particularly, to a nuclear fusion reactor which comprises a forcibly-cooled type reactor wall having low tritium permeability and which uses hydrogen isotope fuel, the forcibly-cooled type reactor wall being positioned to confront plasma or corpuscular rays.
As disclosed in Japanese Patent Unexamined Publication No. 61-104064, a conventional nuclear fusion reactor has been constituted such that a low tritium-permeable layer is formed on the surface of a cooling metal base of a nuclear fusion reactor wall confronting hydrogen isotope plasma for the purpose of preventing the penetration of tritium. The thus formed low tritium-permeably layer has been directly exposed to plasma corpuscles. A metal layer formed on the surface of the cooling metal base provided for the same purpose and having a large recombination constant of the hydrogen isotope has also been exposed to plasma corpuscles as disclosed in Japanese Patent Unexamined Publication No. 62-240773.
As disclosed in Japanese Patent Unexamined Publication Nos. 61-99667 and 56-115983, a structure has constituted such that the tritium penetration preventive layer is formed on the surface of the cooling metal base adjacent to the cooling medium so that the low tritium-permeable layer is not exposed to the plasma corpuscles. According to a structure of the type described above, the cooling metal base is directly exposed to plasma corpuscles. Therefore, a problem takes place in that the soundness of the water cooling metal base deteriorates such that the surface can be fused, and hydrogen embrittlement can take place. On the other hand, a structure disclosed in Japanese Patent Unexamined Publication No. 59-151084 and arranged such that the cooling metal base is covered with a member made of a high fusing point heat resistant material such as SiC so as to be protected from plasma has been constituted such that no penetration-preventive layer is provided between the heat resistant member and the passage through which the cooling medium passes through. Furthermore, the portion of the cooling metal base covered with no heat resistant member has no low tritium permeable layer formed thereon.
According to the above-described conventional structures, the low tritium-permeable layer formed on the surface of the cooling metal base exposed to the plasma for the purpose of preventing the penetration of tritium is subjected to sputtering of hydrogen isotope or impure corpuscles emitted from the reactor core. Furthermore, the surface layer of the base is fused due to a heat load, causing the applied layer to be broken. Furthermore, another problem arises in that the diffusional speed of the hydrogen isotope is raised excessively due to the rise of the temperature of the low-tritium penetration preventive layer and the quantity of the tritium penetration is thereby enlarged excessively. In this case where the tritium penetration preventive layer is provided on only the surface of the cooling metal base adjacent to the passage through which the cooling medium passes, hydrogen isotopes made incident upon the surface of the metal base confronting the plasma can be easily diffused and dissolved in the metal base. The thus diffused and dissolved hydrogen isotope segregates in the grain boundary, causing an intergranular fracture to occur. Therefore, a problem takes place in the structural reliability. According to the above-described conventional structures, atoms existing in the outer layer of the cooling base are forcibly ejected or the atoms existing in the outer layer of the same are evaporated due to the collision of plasma corpuscles. As a result, a problem arises in that the temperature of the plasma is lowered since the radiant energy loss effect can be raised due to the introduction of the evaporated atoms from the surface layer into the reactor core plasma.
According to the structure arranged such that the heat resistant member is disposed on the surface of the cooling metal base exposed to the plasma so as to prevent the direct incidence of the plasma corpuscles into the surface of the metal base, hydrogen isotopic molecules or hydrogen isotopes in the form of atoms are dissolved into the base through the surface of the cooling metal base. The thus dissolved hydrogen isotopic molecules or hydrogen isotopes in the form of atoms are easily diffused in the cooling base and penetrate the same, causing a problem in that the hydrogen isotopic molecules or hydrogen isotopes in the form of atoms leak in the cooling medium.
Furthermore, since the solubility of hydrogen isotopes in copper or stainless steel used as material for the cooling metal base rises proportionally to elevation degree of the temperature, a heating method used as means for promoting the redischarge and removal of the hydrogen isotopes which has been temporarily dissolved in the cooling metal cannot overcome the above-described problem. Therefore, in the case where the structure has no low tritium permeable layer for preventing the dissolution of the hydrogen isotopes in the cooling metal base and in the structural material for a vacuum container wall, the quantity of the hydrogen isotope dissolved in the cooling metal base and in the structural material for a vacuum vessel wall is increased during the operation of the reactor core for a long time. As a result, a maintenance and operational problem arises in that the dissolved hydrogen isotopes cannot be easily removed.